Asymmetric synthesis of novel isoindolines: Azasaccharide mimics as potential enzyme inhibitors

Article abstract of DOI:10.1211/0022357011776351

2′,3′-Dideoxy-2′,3′-didehydronucleosides and azasaccharides are known to possess antiviral activity. The synthesized 1-methoxyisoindoline system (10), which is related to the above nucleosides, is potentially stable in-vivo. The 1-methoxyisoindoline was synthesized from the achiral phthalaldehyde in 10 steps via an enantiomerically pure diol obtained by Sharpless asymmetric dihydroxylation. The new heterocyclic compound is an azosaccharide mimic which provides an access to a new series of nucleoside analogues with potential as antiretroviral agents (anti-HIV) and as glycosidase inhibitors.

Full text of DOI:10.1211/0022357011776351

Journal of
Pharmacy and Pharmacology
JPP 2001, 53: 945–948
# 2001 The Authors
Received December 19, 2000
Accepted April 4, 2001
ISSN 0022-3573
Asymmetric synthesis of novel isoindolines:
azasaccharide mimics as potential enzyme inhibitors
D. F. Ewing, C. Len, G. Mackenzie, J. P. Petit, G. Ronco and P. Villa
Abstract
2h,3h-Dideoxy-2h,3h-didehydronucleosides and azasaccharides are known to possess antiviral
activity. The synthesized 1-methoxyisoindoline system (10), which is related to the above
nucleosides, is potentially stable in-vivo. The 1-methoxyisoindoline was synthesized from the
achiral phthalaldehyde in 10 steps via an enantiomerically pure diol obtained by Sharpless
asymmetric dihydroxylation. The new heterocyclic compound is an azosaccharide mimic which
provides an access to a new series of nucleoside analogues with potential as antiretroviral
agents (anti-HIV) and as glycosidase inhibitors.
Introduction
Aminosugars, in which the pyranose or furanose ring oxygen is replaced by a
nitrogen, can have a variety of biological activities; examples include the
glucosidase inhibitor, castanospermine (II; Figure 1) (Look et al 1993) and the
antiviral compound, 1-deoxynojirimycin (I) (Van den Broek 1997). Azasugars also
inhibit HIV replication by altering the glycosylation of the HIV-1 envelope
glycoprotein gp120 and gp41 (Van den Broek 1997). This may be due to the
disrupting effect that aminosugars have on the biosynthesis of oligosaccharides in
the cell membrane. Recently, the nucleoside analogue III, based on 1,3-dihydro-
benzo[c]furan, has been synthesized (Ewing et al 1999, 2000) as an analogue of 2h,3h-
didehydro-2h,3h-dideoxythymine IV (d4T, stavudine, a commercial anti-HIV
nucleoside approved by the US FDA) (Balzarini et al 1987). This novel d4T
analogue has the 2h,3h double bond incorporated into a benzene ring. This modi-
fication retains a phosphorylation site, has enhanced lipophilicity compared with
d4T, but the substrate is likely to be more resistant to the hydrolytic process that
contributes to the short half-life of d4T in-vivo. Furthermore, this system is clearly
very rigid. It has been speculated (Harte et al 1991) that the conformational
restriction imposed by the double bond in d4T is an important factor in its inter-
action with viral enzymes. Nucleoside analogues with conformational rigidity
imposed by cyclopropanation of a furanose or carbocyclic ring have been reported
recently (Marquez et al 1996; Chun et al 2000).
Department of Chemistry,
University of Hull, Hull, HU6
7RX, UK
D. F. Ewing, G. Mackenzie
Laboratoire des Glucides,
Universite de Picardie, F-80039
Amiens, France
C. Len, J. P. Petit, G. Ronco,
P. Villa
Initial biological test results for pyrimidine nucleoside analogues based on 1,3-
dihydrobenzo[c]furan (unpublished data) have shown no anti-HIV activity.
Changing the glycone ring heteroatom from oxygen to nitrogen is one obvious way
to expand the chemotherapeutic potential of this system. Thus the isoindoline
system V with a nitrogen atom in the ring will have different ring dimensions, a
different charge distribution around the ‘glycosidic’ bond and different hydrogen
Correspondence: C. Len,
Laboratoire des Glucides,
Universite de Picardie, F-80039
Amiens, France.
945

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D. F. Ewing et al
1
the H spectra were made by detailed analysis using
decoupling or correlation techniques where appropriate.
Diastereoisomer ratios were determined from the in-
tegration of suitable peaks. Column chromatography
was performed on silica gel (230–400 mesh; Prolabo)
and TLC on silica gel 60, F254 (Merck) with detection by
UV absorbance or phosphomolybdic acid. MeOH–
NH3 is methanol saturated with ammonia gas at room
temperature.
(1S)-1-Azido-2-benzoyloxy-1-[2-(1,3-dioxan-2-
yl)phenyl]ethane (4)
Mesyl chloride (6.40 g, 56 mmol) was added drop-wise
to a stirred solution of (1R)-2-benzoyloxy-1-[2-(1,3-
dioxan-2-yl)phenyl]ethanol (2; Figure 2) (9.2 g,
28 mmol) in pyridine (60 mL) at 0mC. After 12 h, volatile
matter was evaporated under reduced pressure and then
lithium azide (2.25 g, 46 mmol) in dimethylformamide
(DMF; 100 mL) was added and the solution heated at
100mC for 3 h. After the addition of saturated aqueous
NH4Cl (100 mL) the mixture was extracted with CH2Cl2,
the extract worked up and the crude product purified
by column chromatography (hexane–EtOAc, 90:10) to
give 4 (76%) as a white solid, mp 68–72mC; δH (CDCl3)
1.46, 2.27, 4.02, 4.29 (6 H, m, dioxanyl), 4.42 (1 H, dd,
J l 9.1, 11.4, CH2O), 4.73 (1 H, dd, J l 3.8, 11.4,
CH2O), 5.72 (1 H, s, dioxanyl), 5.78 (1 H, dd, J l 3.8,
9.1, CHN3), 7.53 (9 H, m, aromatic H); δC (CDCl3) 26.1,
67.9, 101.9 (dioxanyl), 60.8 (CN3), 68.3 (C-2), 128.9,
130.2, 133.6 (Ph), 128.0, 128.1, 129.0, 129.9, 134.8, 136.8
(aromatic C). Calculated for C19H19O4N3 : C, 64.58; H,
5.42; N, 11.89. Found: C, 64.71; H, 5.50; N, 11.73.
Figure 1 Structures of aminosugars I, II, IV and 2h-3h-dideoxy-
nucleosides III, IV.
bonding possibilities. These factors are likely to have
significant effect on the nature of an enzyme–substrate
interaction of the isoindoline system when incorporated
as a saccharide mimic in a nucleoside analogue.
We report here the first synthesis of the isoindoline
system with 1,3 disposed hydroxy and hydroxymethyl
substituents. This arrangement of substituents parallels
that found in the 1,3-dihydrobenzo[c]furan glycone in-
corporated in III and mimics the crucial features of all
saccharide glycones. Our strategy involves ring closure
of a suitably protected amino aldehyde. Although aza-
saccharides have been obtained by a similar route
(Huang et al 1993), the only example where the iso-
indoline system has been obtained in this way involves
formylation of a protected benzylamine (Simig 1990), a
methodology which is of limited application since the 1-
hydroxyindoline products are unstable and easily revert
back to the amino aldehyde. Furthermore our novel
approach to the isoindoline system is based on an asym-
metric synthesis which leads to an enantiomerically pure
product, (3S)-2-acetyl-3-acetoxymethyl-1-methoxyiso-
indoline, starting from the convenient, inexpensive,
achiral substrate, phthalaldehyde.
(2S)-2-Azido-2-[2-(1,3-dioxan-2-yl)phenyl]ethanol
(7)
A mixture of 4 (6.50 g, 18.4 mmol) was stirred in
MeOH–NH3 (300 mL) for 16 h at room temperature.
Volatile matter was evaporated under reduced pressure
and the residue purified by column chromatography
(hexane–EtOAc, 60:40) to give 7 as an oil (95%); δH
(CDCl3) 1.36, 2.19, 3.91, 4.21 (6 H, m, dioxanyl), 3.68
(2 H, m, CH2O), 5.39 (1 H, dd, J l 4.8, 8.0, CHN3), 5.61
(1 H, s, dioxanyl), 7.35–7.51 (4 H, m, aromatic H); δC
(CDCl3) 26.1, 68.0, 101.8 (dioxanyl), 63.8 (CN3), 66.6
(C-2,), 127.8, 128.1, 128.6, 129.9, 135.7, 136.7 (aromatic
C). Calculated for C12H15O3N3 : C, 57.82; H, 6.06;
Materials and Methods
Chemistry
NMR spectra were recorded with a Lambda 400 spec- N,16.86. Found: C, 57.93; H, 6.33; N, 16.62.
trometer using standard conditions with a data point
resolution of ca. 0.1 Hz. 1H Chemical shifts were (2S)-2-Amino-2-[2-(1,3-dioxan-2-yl)phenyl]ethanol
measured relative to Me4Si and 13C chemical shifts (8)
relative to CDCl3 (77.0 ppm) or (CD3)2SO (39.5 ppm). A mixture of 7 (3.80 g, 15.2 mmol) and PPh3 (4.40 g,
All coupling constants are given in Hz. Assignments of 16.8 mmol) in THF (tetrahydrofuran; 100 mL) was

947
Asymmetric synthesis of novel isoindolines
stirred at room temperature for 24 h. Volatile matter
was evaporated under reduced pressure and the residue
taken up in MeOH–NH3. After 24 h at room temper-
ature, the volatile matter was evaporated under reduced
pressure and the residue was purified by column chro-
matography (CHCl3–MeOH, 70:30) to give 8 as an oil
(54%); δH (CDCl3) 1.44, 2.23, 3.98, 4.25 (6 H, m,
dioxanyl), 3.65 (1 H, dd, J l 4.9, 10.7, CH2O), 3.73
(1 H, dd, J l 8.2, 10.7, CH2O), 4.53 (1 H, dd, J l 4.9,
8.1, CHNH2), 5.68 (1 H, s, dioxanyl), 7.57–7.31 (4 H,
m, aromatic H); δC (CDCl3) 26.0, 67.9, 68.0, 101.0
(dioxanyl), 52.5 (CHNH2), 67.6 (C-2h), 126.4, 127.2,
127.7, 129.8, 136.3, 141.4 (aromatic C).
(1S)-1-Acetylamino-2-acetoxy-1-[2-(1,3-dioxan-2-
yl)phenyl]ethane (9)
A mixture of 8 (504 mg, 2.26 mmol) and anhydride
acetic acid (1.27 mL) in pyridine (10 mL) was stirred at
room temperature for 12 h. Volatile matter was evapo-
rated under reduced pressure and the residue was puri-
fied by silica gel column chromatography (CHCl3–
MeOH, 90:10) to give 9 as an oil (91%); δH (CDCl3):
1.38, 2.17, 4.0, 4.18 (6 H, m, dioxanyl), 1.88, 1.99 (6H, s,
COCH3), 4.28 (2 H, m, CH2O), 5.61 (1 H, m, CHNHAc),
5.80 (1 H, s, dioxanyl), 7.26–7.61 (4 H, m, aromatic H);
δC (CDCl3) 21.3, 23.5 (CH3), 26.0, 67.7, 99.6 (dioxanyl),
48.8 (CHNHAc), 66.3 (C-2), 126.8, 127.0, 128.3, 129.4,
136.8, 136.9 (aromatic C), 170.2, 171.7 (CO).
Figure 2 Synthesis of (3S)-3-acetoxymethyl-2-acetyl-1,3-dihydro-
1-methoxyisoindole (10). Reagents and conditions: i, Ewing et al
(2000); ii, MsCl, pyridine; iii, LiN3, DMF; iv, PPh3, NH3, THF; v,
NH3, MeOH; vi, Ac2O, pyridine; vii, HCl 1%, MeOH.
(C-2), 90.4 (CHOMe), 123.4, 129.1, 129.7, 130.3, 135.7,
139.0 (aromatic C), 170.9, 172.6 (CO).
Results and Discussion
The protected dihydroxyaldehyde 2 was easily prepared
(Figure 2) from phthalaldehyde (1) (Ewing 1999, 2000)
in four steps including asymmetric dihydroxylation with
(3S)-3-Acetoxymethyl-2-acetyl-1,3-dihydro-1-
methoxyisoindole (10)
Compound 9 was dissolved in methanolic HCl (1%, Admix β. The dihydroxylation step is completely stereo-
10 mL) and the mixture stirred for 2 h at room tem- selective, which is important in generating the R con-
perature. Water was added and the mixture extracted figuration at the chiral centre (which will become part of
with diethyl ether. The extract was worked up and the the new ring). The essential strategy to get an appro-
crude product purified by column chromatography priate nitrogen functionality in place requires the in-
(CHCl3–MeOH, 99:1) to give 10 (34%) (cis and trans sertion, with inversion, of an azido group at the chiral
1:1) as an oil; cis isomer, δH (CDCl3) 1.92, 2.25 (6H, two site, and this was achieved by O-mesylation followed by
s, COMe), 3.1 (3H, s, OMe), 4.39 (1 H, dd, J l 6.1, 11.0, treatment with lithium azide in dimethylformamide to
CH2O), 4.67 (1 H, dd, J l 4.0, 11.0, CH2O), 5.28 (1 H, give intermediate 4 in 76% yield. Reduction of the azido
dd, J l 4.1, 6.0, CHNHAc), 6.29 (1 H, s, CHOCH3), group in 4 was best accomplished with the Staudinger
7.35 (4 H, m, aromatic H); δC(CDCl3) 21.2, 22.7 procedure using PPh3–NH3 in methanol. Under these
(COMe), 50.5 (OMe), 60.6 (CHNHAc), 65.1 (C-2), 91.2 conditions, an O-to-N migration of the benzoyl group
(CHOMe), 123.8, 124.5, 129.0, 130.4, 135.8, 139.2 (aro- occurred to some extent and a mixture of the ester 5 and
matic C), 170.9, 172.6 (CO); trans isomer, δH (CDCl3) the amide 6 was obtained. This mixture was not amen-
1.79, 2.27 (6H, s, COMe), 2.77 (3H, s, OMe), 4.53 (1 H, able to separation by column chromatography. How-
dd, J l 6.1, 11.0, CH2O), 4.67 (1 H, dd, J l 4.0, 11.0, ever, this problem could be avoided by removing the
CH2O), 5.4 (1 H, dd, J l 4.1, 6.0, CHNHAc), 6.41 (1 H, protecting benzoyl group before reduction. Thus, the
s, CHOMe), 7.35 (4 H, m, aromatic H); δC (CDCl3) 21.0, azido alcohol 7 was reduced (54% yield), and the
23.0 (2C, COMe), 48.6 (OCH3), 62.2 (CHNHAc), 63.7 primary hydroxy group reprotected, by acetylation.

948
D. F. Ewing et al
comparison with their parental 2h,3h-dideoxyribonucleosides. Mol.
Pharmacol. 32: 162–167
Removal of the aldehyde protection led to spontaneous
cyclization to give (3S)-3-acetoxymethyl-2-acetyl-1,3-
dihydro-1-methoxyisoindole 10 as a pair of diastereo-
isomers. The presence of a 1-methoxy group prevents
both reversion to the corresponding amidoaldehyde and
1,3 dehydration to an isoindole.
Chun, B. K., Olgen, S., Hong, J. H., Newton, M. G., Chu, C. K.
(2000) Enantiomeric syntheses of conformationally restricted -
and -2h,3h-dideoxy-2h,3h-endo-methylene nucleosides from carbo-
hydrate chiral templates. J. Org. Chem. 65: 685–693
Ewing, D. F., Fahmi, N. E., Len, C., Mackenzie, G., Ronco, G., Villa,
P., Shaw, G. (1999) Nucleoside analogues with a novel glycone
based on the benzo[c]furan core. Nucleosides Nucleotides 18: 2613–
2630
Compound 10 has the potential to behave as an
azasaccharide and hence couple with a silylated nucleo-
base using trimethylsilyl triflate as a catalyst (the Vor-
bruggen procedure) to provide access to novel nucleo-
side analogues. The bioassay of compound 10 and
related nucleoside analogues will be reported elsewhere.
In conclusion, the 1-methoxyisoindoline derivatives
(10) possessing two asymmetric carbon atoms were
conveniently prepared in good overall yield starting
from achiral phthalaldehyde. A stereoselective method
employing asymmetric dihydroxylation gave stable
enantiomerically pure azasaccharides. These com-
pounds have potential biological activity and could be
used to form novel glycone moieties in nucleoside
chemistry.
Ewing, D. F., Fahmi, N. E., Len, C., Mackenzie, G., Pranzo, A.
(2000) Stereoisomeric pyrimidine nucleoside analogues based on
the dihydrobenzo[c]furan core. J. Chem. Soc. Perkin Trans. 1:
3561–3565
Harte, W. E., Starrett, J. E., Martin, J. C., Mansuri, M. M. (1991)
Structural studies of the anti-HIV agent 2h,3h-didehydro-2h,3h-di-
deoxythymidine (d4T). Biochem. Biophys. Res. Commun. 175: 298–
304
Huang, B., Chen, B., Hui, Y. (1993) First synthesis of 4h-acetamido-
2h-deoxythymidine. Synthesis 8: 769–771
Look, G. C., Fotsch, C. H., Wong, C. H. (1993) Enzyme-catalyzed
organic synthesis: practical routes to aza sugars and their analogs
for use as glycoprocessing inhibitors. Acc. Chem. Res. 26: 182–190
Marquez, V. E., Siddiqui, M. A., Ezzitouni, A., Russ, P., Wang, J.,
Wagner, R. W., Matteucci, M. D. (1996) Nucleosides with a twist.
Can fixed forms of sugars ring pucker influence biological activity
in nucleosides and oligonucleotides? J. Med. Chem. 39: 3739–3747
Simig, G. (1990) A new route to isoindole intermediates. Synlett
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